I. Field of the Invention
The present invention relates to improved pressure sensitive adhesives characterized by superior cohesive strength, aggressive tack, load bearing ability, heat resistance and resistance to solvents. More particularly, the present invention is directed to pressure sensitive adhesive compositions comprising a mixture of an acrylic pressure sensitive polymer, a chlorosulfonated polyethylene and a polymerizable vinyl compound. The compositions are cured to form a strong bond upon contact with a condensation reaction product of a primary or secondary amine and an aldehyde.
II. Brief Description of the Prior Art
Pressure sensitive adhesives, particularly acrylate-based pressure sensitive adhesives, are used for a wide variety of sealing, holding, fastening, mending and masking purposes due to their ability to form a bond at room temperature immediately upon contact, merely by the application of light pressure.
The major disadvantage to the use of the pressure sensitive adhesives is that, by their chemical nature, they have inherently low cohesive strength. In addition, they are thermoplastic and soften rapidly at temperatures in excess of about 140.degree. F.; hence they are limited to applications where the stress on the bond or assembled joint is low (i.e., generally less than 2 psi in shear) and where temperature exposure not much in excess of room temperature is expected under this stress. In addition, pressure sensitive adhesives generally soften and cannot be used where exposure to commonly encountered organic fluids, such as gasoline, lubricating oils, transformer oils and industrial cleaning compounds is expected.
Various attempts have been undertaken to improve these inherent weaknesses of pressure sensitive adhesives, thus permitting expanded use thereof. As an example, the adhesive may be vulcanized or crosslinked during preparation of the pressure sensitive coated article such as described in U.S. Pat. Nos. 2,410,053, 2,973,286 and 3,707,518. These treatments can offer only small improvement in strength if they are to avoid damaging the aggressive tack required of a pressure sensitive adhesive. Additionally, these treatments often require an extended drying cycle in terms of time and energy required as well as at temperatures which may distort or embrittle the articles being coated. In other instances, the inherent weaknesses of pressure sensitive adhesives have been overcome by crosslinking the adhesive once the pressure sensitive article has been bonded (i.e., the final assembly has been made). Examples of such procedures are described in U.S. Pat. Nos. 3,118,534 and 3,307,690. In all cases, such procedures generally involve cure temperatures in excess of 100.degree. C. for periods of time varying from 30 minutes to several hours. Often such cures are not feasible due to the size of the assembled part or the elevated temperature sensitivity of various components of the assembly.
It is therefore an object of the present invention to provide a pressure sensitive adhesive composition which has aggressive tack for convenient application at room temperature.
It is a further object of the invention to provide an adhesive composition which cures or crosslinks subsequent to final assembly, the crosslinking being affected quickly and at room temperature in a manner such that the load bearing ability, heat resistance and resistance to various organic fluids is improved.